1. Field of the Invention
The present invention relates to a charging apparatus and an image forming apparatus.
2. Description of the Related Art
In electrophotographic image forming apparatus such as copying machines, printers and facsimile units, images are formed by using a photoreceptor in which a photosensitive layer containing a photoconductive substance on the surface is formed as an image carrier, and applying charges to the surface of the photoreceptor to uniformly charge the same, forming electrostatic latent images corresponding to image information by various image forming processes, developing the electrostatic latent images by a developer containing a toner supplied from developing means into visible images, transferring the visible images on a recording material such as paper, and heating and pressing them by a fixing roller thereby fixing them on a recording material thereby forming images on a recording paper.
In the image forming apparatus described above, a charging apparatus is used for charging the surface of the photoreceptor. The charging apparatus includes, usually, a charging wire (discharging wire) as an electrode for conducting corona discharge, a grid electrode applied with an appropriate voltage and controlling the amount of charges applied by the charging wire to the surface of the photoreceptor, that is, the charged potential on the surface of the photoreceptor, and a support member for supporting the charging wire and the grid electrode Then, as the grid electrode, a wire grid electrode prepared from stainless steel or tungsten, a porous plate grid electrode in which a plurality of perforations are formed to a metal plate formed of a stainless steel or the like (grid substrate), etc. are used. Upon manufacturing the porous plate grid electrode, etching or other methods can be adopted for aperturing the perforations to the metal plate. The porous plate grid electrode manufactured by etching is referred to as an etching grid.
Among these grid electrodes described above, the wire grid electrode involves a problem to be solved that contaminants such as a toner are tended to be deposited, and the function for controlling the charged potential on the surface of the photoreceptor becomes insufficient by the deposition of the contaminants, making the charged potential not uniform on the surface of the photoreceptor.
On the other hand, since the porous plate grid electrode has a relatively larger area compared with the wire grid electrode, the charged potential on the surface of the photoreceptor can be controlled to a proper range and, in addition, the controllability for the charged potential is less lowered even when the contaminants are deposited more or less. Further, since the porous plate grid electrode is formed of iron series metal materials such as stainless steel as described above, it has high durability, causes no disadvantage such as deformation even after long time use, and the change of the controllability for the charged potential due to deformation or the like is extremely small. Accordingly, it is considered that the porous plate grid electrode can control the charged potential on the surface of the photoreceptor substantially constant for a long time. However, while the iron series metal material such as stainless steel as the material for the porous plate grid electrode usually has high durability, it involves a drawback that it is oxidized easily due to water content under a high humidity circumstance and ozone generated upon corona discharge during charging operation. In the use of the porous plate grid electrode for a long time, use under the high humidity circumstance and contact with ozone are inevitable. Accordingly, in the porous plate grid electrode formed from a metal material such as stainless steel, corrosion such as rust occurs due to the water content in air, ozone, etc. and nitrogen oxides are deposited on the surface thereof to deteriorate the durability. In addition, the controlling performance for the charged potential in the photoreceptor becomes insufficient, and it involves a problem to be solved that the charged potential becomes not uniform on the surface of the photoreceptor and a desired charged potential can not be applied always stably to the surface of the photoreceptor.
In view of the problems described above in the porous pate grid electrode, a corona charging apparatus having a grid electrode formed by successively coating a nickel plating layer and a gold plating layer on the surface of a metal plate formed from a stainless steel (grid substrate) has been proposed (for example, refer to Japanese Unexamined Patent Publication JP-A 11-40316(1999)). In the grid electrode, improvement is indeed recognized to some extent in view of the corrosion resistance but it involves a problem that the gold plating layer mainly attributable to the enhancement of the corrosion resistance is tended to be peeled. While the nickel plating layer is formed between the metal plate and the gold plating layer in order to prevent peeling of the plating layer in JP-A 11-40316, the effect is not yet satisfactory.
Further, a charging apparatus having a grid electrode formed directly with a gold plating layer by an electrolytic plating method using a pulse current not by way of a nickel plating layer on the surface of a metal plate formed from a stainless steel (grid substrate) has been proposed (for example, in Japanese Unexamined Patent Publication JP-A 2001-166569). The charging apparatus is used in an image forming apparatus including an organic photoreceptor to be formed with electrostatic images, a charging apparatus for charging the organic photoreceptor, developing means for developing electrostatic latent images formed on the organic photoreceptor into toner images, a transfer portion of transferring the toner images on a recording material, and fixing means for fixing the toner images on the recording material. In the charging apparatus, since the gold plating layer is less peeled, the corrosion resistance and the controllability for the charged potential on the surface of the photoreceptor are satisfactory in the grid electrode. By the way, it is necessary to form the gold metal layer to a thickness of 0.3 μm or more in order that the grid electrode can provide the preferred characteristics sufficiently. In addition, since the grid electrode is a relatively large member having a substantially the same size as the photoreceptor, the amount of gold to be used is necessarily increased also because of the necessity for increasing the thickness of the plating layer. However, use of gold in such a great amount unnecessarily increases the cost of the charging apparatus and, thus, impairs the general applicability of the image forming apparatus due to the relatively reduced cost, which is one of the advantages of the image forming apparatus. Accordingly, it has been demanded for a charging apparatus not using an expensive material such as gold and excellent in the durability and the controllability for the charged potential on the surface of the photoreceptor.
Further, the charging apparatus comprises an electrode for conducting corona discharge to the photoreceptor, a grid electrode as an electrode which is disposed optionally between the surface of the photoreceptor and the electrode for controlling amount of charges applied to the surface of the photoreceptor, that is, the charged potential on the surface of the photoreceptor, and a support member for supporting the electrode and the grid electrode. Since the grid electrode can control the charged potential on the surface of the photoreceptor substantially accurately, the charging apparatus provided with the grid electrode has become predominant at present. For the grid electrode, a wire grid electrode formed, for example, from stainless steel or tungsten, and a porous plate grid electrode in which a plurality of perforations are formed to a metal plate formed, for example, from stainless steel (grid substrate), etc are used.
As an electrode for use in the charging apparatus are used a wire electrode, a metal plate electrode having a plurality of needle portions (hereinafter referred to as “needle electrode”), etc. Among them, the needle electrode having advantages of requiring less number of constituent parts, having longer life, with less generation amount of ozone and with less failure because disconnection does not occur is preferably used. The needle electrode is manufactured by applying etching to a metal plate mainly formed, for example, from iron series metal material such as stainless steel to form a plurality of needle portions. The needle electrode manufactured by etching is also referred to as an etched electrode. Since the etched cross section of the needle electrode lacks in the smoothness and a plurality of edges for conducting discharge are present at the top end of the needle portion, as well as the shape of the edges present at the top ends of a plurality of needle portions is not uniform, discharging at each of the needle portions become not uniform. As a result, the charged potential on the surface of the photoreceptor can not be controlled sufficiently to make the charged potential not uniform on the surface of the photoreceptor.
Further, while the iron series metal material such as stainless steel as the material for the needle electrode has high durability, it involves a drawback to be oxidized easily due to water content under a high humidity circumstance and ozone generated by corona discharging during charging operation. Then, in the use of the needle electrode for a long time, use under the high humidity circumstance, contact with ozone, etc. are inevitable. Accordingly, in the needle electrode formed from a metal material such as stainless steel, corrosion occurs due to moisture in air and ozone to deteriorate the durability. In addition, the controlling performance for a high voltage applied to the needle electrode for generating corona discharge from the needle portion is lowered and the charged potential becomes not uniform on the surface of the photoreceptor to result in a problem to be solved that a desired charged potential can not always be applied stably to the surface of the photoreceptor.
Further, the wire electrode also involves a problem to be solved similar to that in the needle electrode that rust or corrosion occurs due to ozone generated by corona discharge and the charged potential on the surface of the photoreceptor becomes not uniform.
In view of the foregoing problems in the charging apparatus, it has been proposed, for example, a charging apparatus including a wire electrode extended in a shield case opened at one surface and a plate grid electrode arranged between the wire electrode and the photoreceptor, wherein the plate grid electrode is formed by applying a nickel plating layer of about 1 μm thickness on the surface of a porous stainless steel plate and forming a gold plating layer of about 0.3 μm thickness further thereon is provided (for example in JP-A 11-40316). In the plate grid electrode of JP-A 11-40316, since the gold plating layer is formed by way of the nickel plating layer, the gold plating layer is less peeled and the corrosion resistance and the controllability for the charged potential on the surface of the photoreceptor are relatively satisfactory. However, since manufacture of the plate grid electrode requires plating steps twice, i.e., nickel plating and gold plating, the manufacturing step is complicated to increase the cost. Further, it is necessary to form the gold metal layer to a thickness of 0.3 μm or more in order that the plate grid electrode can provide the preferred characteristics sufficiently. In addition, since the plate grid electrode is a relatively large member having a substantially the same size as the photoreceptor, the amount of gold to be used is necessarily increased also because of the necessity for increasing the thickness of the plating layer. However, use of gold in such a great amount unnecessarily increases the cost of the charging apparatus and, thus, impairs the general applicability of the image forming apparatus due to the relatively reduced cost, which is one of the advantages of the image forming apparatus. Accordingly, it has been demanded for a charging apparatus having the needle electrode and the plate grid electrode not using an expensive material such as gold and excellent in the durability and the controllability for the charged potential on the surface of the photoreceptor.
Further, JP-A 2001-166569, for example, also proposes a charging apparatus having a wire electrode and a plate grid electrode in which a gold plating layer is formed by an electrolytic plating method using a pulse current directly on the surface of the stainless steel metal plate similar to that in JP-A 11-40316. Also in this plate grid electrode, the gold plating layer hardly peels and, like the plate grid electrode in JP-A 11-40316, the corrosion resistance is high and the controllability for the charged potential on the surface of the photoreceptor is also favorable. However, since it is necessary to increase the thickness of the gold metal layer to 0.3 μm or more, also in the plate grid electrode, it involves the same drawback as in the charging apparatus of JP-A 11-40316.
On the other hand, it has also been proposed to coat gold on the side of the electrode (refer for example to Japanese Unexamined Patent Publication JP-A 2004-4334). The charging apparatus of JP-A 2004-4334 includes a needle electrode in which coating layers comprising gold, platinum, copper, nickel or chromium are formed by plating on the surface thereof. While a method of etching, precision pressing, etc. is used for forming the needle electrode, the cross section of the needle electrode obtained by the method lacks in smoothness and results in fine irregularities. Accordingly, even after applying the plating, fine irregularities on the cross section leaves as it is to disturb the balance of the corona discharge causing the charged potential not uniform on the surface of the photoreceptor. Further, contaminants such as a toner are tended to be deposited on the fine irregularities. That is, since the needle electrode of JP-A 2004-4334 suffers from contaminants such as the toner during long time use, which further makes the charged potential on the surface of the photoreceptor not uniform.